Undesirable excess heat is removed in many industrial processes by the use of heat exchangers in which water is used as the heat exchange fluid. Copper and copper-bearing alloys are often used in the fabrication of such heat exchangers, as well as in other parts in contact with the cooling water, such as pump impellers, stators, and valve parts. The cooling fluid is often erosive and/or corrosive towards these metal parts by virtue of the cooling fluid having high turbidity, aggressive ions, and by the intentional introduction of oxidizing biocides for biological control.
The consequences of such erosion and corrosion are the loss of metal from the equipment, leading to failure or requiring expensive maintenance; creation of insoluble corrosion product films on the heat exchange surfaces, leading to decreased heat transfer and subsequent loss of productivity; and discharge of copper ions which can then “plate out” on less noble metal surfaces and cause severe galvanic corrosion, a particularly insidious form of corrosion. Also, since copper is a toxic substance, its discharge to the environment is undesirable. Prevention or at least minimization of such discharge is a great problem in view of increasingly stringent public attitudes and legislation relating to pollution of the environment.
It is common practice to introduce corrosion inhibitors into the cooling water. These materials interact with the metal to directly produce a film which is resistant to corrosion, or to indirectly promote formation of protective films by activating the metal surface so as to form stable oxides or other insoluble salts. However, such films are not completely stable, but rather are constantly degrading under the influence of the aggressive conditions in the cooling water. Because of this effect, a constant supply of corrosion-inhibiting substances must generally be maintained in the cooling water. A constant depletion of such substances occurs because many cooling systems are open, requiring continuous addition of fresh water to compensate for evaporation and blowdown (i.e., discharge). Continuous addition of fresh corrosion-inhibiting substances is likewise required so as to maintain, within defined limits, a concentration of such substances sufficient for the purpose of maintaining good corrosion inhibition. Moreover, currently used materials do not inhibit erosion of the copper-containing surfaces from the effects of particles in high turbidity water in many industrial processes.
Benzotriazole, mercaptobenzothiazole, and tolyltriazole are well known copper corrosion inhibitors. For example, U.S. Pat. No. 4,675,158 discloses the use of tolyltriazole/mercaptobenzothiazole compositions as copper corrosion inhibitors. Also see U.S. Pat. No. 4,744,950, which discloses the use of lower (e.g., C3-C6 linear alkyl, especially n-butyl) alkylbenzotriazoles as corrosion inhibitors and teaches intermittently feeding such compounds to an aqueous system.
U.S. Pat. No. 5,746,947 relates to alkylbenzotriazole compositions comprising a C3-C12 alkylbenzo-triazole and a mercaptobenzothiazole, tolyltriazole, benzotriazole, 1-phenyl-5-mercaptotetrazole, and salts thereof for use as corrosion inhibitors. Additionally, these compositions generally provide improved tolerance to oxidizing biocides such as chlorine and bromine. While 5-(lower alkyl)benzotriazoles are known which do not require continuous feeding in order to inhibit copper corrosion (see U.S. Pat. No. 4,744,950), these compounds provide relatively poor performance in the presence of chlorine, and may be ineffective in both dissolved and suspended high-solids waters. U.S. Pat. No. 5,746,947 teaches that depending on water aggressiveness, the time between feedings may range from several days to months.
U.S. Pat. No. 5,236,626 relates to alkoxybenzotriazole compositions comprising a C3-C12 alkoxybenzotriazole; and a mercaptobenzothiazole, tolyltriazole, benzotriazole, substituted benzotriazoles such as chlorobenzotriazole, nitrobenzotriazole, etc. and 1-phenyl-5-mercaptotetrazole, and salts thereof for use as corrosion inhibitors. These compositions are effective in high-solids water and form long-lasting protective films on metallic surfaces in contact with aqueous systems. Additionally, these compositions generally provide improved tolerance to oxidizing biocides such as chlorine and bromine.
The above deficiencies are generally overcome by the instant compositions. It is therefore an object of the instant invention to provide erosion inhibitors that also produce more halogen resistant protective films. The inhibitors are further effective in reducing copper discharge into the environment in high-solids, particularly high dissolved solids, aggressive waters.